This plant is good to pick in May while still tender. The leaves are a rather 'greyish' kind of green, downy to the touch and rip easily. The plant is easy (too easy!) to pull up at this stage. It is best to use is as a 'cut and come again' plant, as later in the year when it grows to 2 or 3ft tall and produces multiple tiny pale yellow dandelion-like flowers, it becomes tougher and stringy and the leaves are more bitter.

As ever, always wash leaves thoroughly if picked from outside your own garden.

 From the PFAF database:

A fairly common garden weed, nipplewort will maintain itself in the garden so long as it is given some disturbed soil in which to self-sow. It is a fairly tolerant plant that succeeds in most soils, so long as they are not too acid, and dislikes heavy shade. It was at one time cultivated as a vegetable. ref: PFAF